In certain high-density information record/playback systems, recorded information appears as an information track constituting relatively short wavelength variation (e.g., 0.6 to 1.6 micrometers) in the geometry of the groove bottom along its length. In one specific but nonlimiting embodiment, this short wavelength variation may encode, for example, a composite color video signal. There are several methods of encoding a composite color video signal in the information track comprising short wavelength variation. Illustratively, the method of encoding may be of the type shown in the copending U.S. Patent Application of E. O. Keizer, Ser. No. 441,069, filed Feb. 11, 1974, entitled, COLOR PICTURE/SOUND RECORD AND RECORDING/PLAYBACK APPARATUS AND METHODS THEREFOR, and now U.S. Pat. No. 3,911,476. Pursuant to the Keizer method, a first (e.g., video) carrier is frequency modulated over a high frequency deviation range (e.g., of the order of 4.3 to 6.3 MHz) in accordance with a video signal including the luminance and the chrominance of a scanned image. A second carrier is frequency modulated over a low frequency deviation range (e.g., of the order of 716 KHz .+-. 50 KHz) in accordance with the audio signal accompaniment of the video signal. The once-modulated first carrier is duty cycle modulated in accordance with the once-modulated second carrier. A scanning electron beam apparatus responsive to the twice modulated first carrier records in the groove bottom of a pregrooved master having a coating of photoresist material short wavelength variation representative of the time variation of the recorded signal.
Ordinarily, a stamper (having negative grooves) is obtained from the recorded master (having positive grooves) from which plastic disc records (also having positive grooves) can then be molded. To reconstruct the prerecorded signals, an appropriate relative motion is established between the grooved disc record and a grooveengaging signal pickup responsive to the spatial variation passing underneath. The signal pickup may be of any suitable variety (for example, a capacitance or a pressure type, etc.). Reference may be made to the U.S. Pat. No. 3,842,194, issued to J. K. Clemens, on Oct. 15, 1974 and entitled INFORMATION RECORDS AND RECORDING/PLAYBACK SYSTEMS THEREFOR, for an illustration of a playback apparatus including a capacitance type of a signal pickup. Pursuant to the Clemens' system, the grooved disc record is provided with a thin deposit of dielectric material overlying a fine coating of conductive material on the base of the disc record (with respective thicknesses sufficiently small that the dielectric deposit and the conductive coating follow the contours of the groove and the groove bottom variation therein). A playback stylus has a groove-engaging tip incorporating a conductive electrode. The disc record is roatated at an appropriate speed in order to cause variation in the capacitance exhibited between the stylus electrode and the disc record conductive coating in accordance with the signal recorded in the groove bottom. A detector responsive to the capacitance variation reconstructs the prerecorded signal for audio/visual presentation on an ordinary television receiver.
In the above-said type of video disc systems, in order to obtain adequate bandwidth for the signal recovered from the grooved disc record (e.g., 4.3 to 6.3 MHz) during playback (1) the disc record is rotated at a relatively high playback speed (e.g., 450 rpm), and (2) the wavelength of the modulation in the disc record groove is relatively short (e.g., 0.6 to 1.6 micrometers) as compared with conventional audio disc systems.
Since the playback time is (1) directly proportional to the number of grooves per inch in the disc record, and (2) inversely proportional to the playback speed of the disc record, the higher playback speed (e.g., 450 rpm) results in a larger number of grooves per inch in the disc record (e.g., 5,555 gpi) for a given playback time (e.g., thirty minutes from each side). In other words, in the aforesaid type of video disc systems, the groove convolutions are very closely spaced (e.g., 4.5 micrometers) in order to accommodate the information necessary for storing a video program of an acceptable quality and a reasonable playback time. The close spacing of the groove convolutions in the video disc type record (e.g., 4.5 micrometers) results in a groove having a very small quiescent (without groove modulation) depth (e.g., 0.8 micrometer with a groove-apex angle of 140.degree.).
It has been determined from the noise spectra of the recorded master that the noise level in the signal recovered from the grooved disc record during playback is reduced as the wavelength of the noise components measured is reduced. In other words, for a given recording level (the peak-to-peak dimension of the groove modulation), the signal-to-noise ratio for a given noise bandwidth improves as the wavelength of the groove modulation due to the recorded signal, and the corresponding noise components, is reduced. With electrical recording in a lacquer master, it has been possible to obtain a satisafctory signal-to-noise ratio (e.g., above 40 dB peak-to-peak video/rms noise) with a low recording level (e.g., 0.1 to 0.15 micrometers) when the wavelength of the groove modulation, due to the recorded signal, and the corresponding noise components, is kept relatively short (e.g., 0.6 to 1.6 micrometers). It is desirable to further improve the signal-to-noise ratio for a given recording level; e.g., to accommodate the unavoidable addition of noise during the video disc type record manufacturing operations.
A variety of approaches to disc recording exist in the prior art. For example, electromechanical processes are known in the audio industry for recording groove modulation representative of an audio signal (e.g., having a bandwidth of 20 KHz) in a lacquer master. The following recording parameters are typical in the audio recording processes: (a) the number of grooves per inch -- 150 to 350; (b) the groove depth -- up to 50 micrometers; (c) the wavelength of the groove modulation -- greater than 10 micrometers and (d) the peak-to-peak dimension of the groove modulation -- up to 80 micrometers. Further, the cutting stylus emloyed for recording audio groove modulation in a lacquer master is heated in order to obtain a satisfactory recording (e.g., a reasonably good signal-to-noise ratio). It has been found that the heated cutting stylus causes the lacquer material to flow while cutting a groove and modulation therein producing a burnishing effect (i.e., surface polish) in the grooves.
A cutterhead for electromechanically recording a video signal in a lacquer master is described in U.S. Pat. No. 3,865,997, issued to J. B. Halter on Feb. 11, 1975, an entitled, TRIANGULAR PIEZOELECTRIC TRANSDUCER FOR RECORDING VIDEO INFORMATION. In the above-said patent No. (3,865,997), the cutting stylus employed for recording groove modulation in a lacquer master for storing a video signal is also shown heated for satisfactory recording (i.e., reducing the surface noise).